This invention relates to methods and apparatus for performing wet oxidation of semiconductor wafers. More particularly, the invention relates to safe methods and apparatus for forming high quality oxides via wet oxidation.
Wet and dry oxidation processes are currently employed by the semiconductor industry for the purpose of forming oxide layers such as gate oxides and isolation oxides on semiconductor surfaces. Dry oxidation processes typically employ molecular oxygen, nitrous oxide, nitric oxide, or some combination thereof to react with a semiconductor substrate surface and produce a layer of semiconductor oxide. The nitrogen containing a species may be employed when it is desirable to impart some nitrogen to the oxide, as is the case with hardened gate oxides, for example. Because it is relatively slow in comparison to wet oxidation processes, dry oxidation is typically limited to the formation of relatively thin oxide films.
Wet oxidation is the subject of the present invention. Wet oxidation involves reacting ultra pure water with the semiconductor surface to form oxide layers. Water vapor is typically reacted with a silicon substrate at a temperature in a neighborhood of 700 degrees Centigrade to form oxide layers between about 50 and 6000 angstroms in thickness. The ultra pure water is typically produced by a xe2x80x9ctorchxe2x80x9d which is a reactor where ultra pure gaseous hydrogen is reacted with ultra pure gaseous oxygen to produce the water.
FIG. 1 presents a cross-sectional diagram of a wet oxidation system 10. The system 10 includes a xe2x80x9ctorchxe2x80x9d II and a xe2x80x9cfurnacexe2x80x9d 13 (terms used in the industry). As mentioned, hydrogen and oxygen react in the torch to form water. The water from torch 11 is piped to furnace 13 where it reacts with the silicon on multiple wafers to form silicon oxide layers.
Torch 11 contains a torch chamber wall 15, which is typically made from quartz. Chamber wall 15 may assume a generally xe2x80x9cjug-shapedxe2x80x9d configuration having a diameter of roughly 2xe2x80x3-12xe2x80x3 and a height of roughly 12xe2x80x3-20xe2x80x3. Generally, the torch is substantially smaller than the furnace. Note that torch 11 and furnace 13 are not drawn to scale in FIG. 1.
Ultra pure hydrogen and oxygen are introduced to the interior of torch 11 through an annular arrangement of quartz pipes. A hydrogen inlet 17 is provided through the bottom of torch chamber wall 15. A quartz inlet is defined by an annular pipe 19 which circumferentially surrounds hydrogen inlet pipe 17. The flow of hydrogen through inlet 17 is controlled by the size of an orifice 21 which may be a constriction at the end of quartz inlet pipe 17.
Torch 11 also includes a heater 23 which jackets a portion quartz vessel 15. Heater 23 may in one embodiment be a simple coil heater. It provides the energy necessary to ignite a hydrogen-oxygen flame 25 within the interior of torch 11. In many designs, the interior of torch 11 is maintained at a temperature of about 540 to 700 degrees Centigrade. At atmospheric pressure, a temperature of about 540 degrees Centigrade is required for ignition. The hydrogen-oxygen reaction is highly exothermic, producing a flame temperature in the neighborhood of 5000 degrees Centigrade. The water produced by the reaction of hydrogen and oxygen in flame 25 is directed to the bottom of furnace 13 through a pipe 27. There, it passes up through an injection tube 29 which is typically a hollow quartz tube oriented vertically in the center of the furnace.
Furnace 13 also includes a container wall 31 which may be made from quartz. It is generally cylindrically shaped and has a diameter of roughly 10xe2x80x3 and a height of roughly 45xe2x80x3. Wall 31 is jacketed by a very precise heater 33 which supplies sufficient energy to drive the oxidation reaction. To careful control the temperature within furnace 13, multiple thermocouples may provided in close proximity to wafers 35. Other thermocouples are provided in heater 33. Typically, the furnace interior is maintained at a temperature of about 700 or more degrees Centigrade. Water from water injection column 29 disperses throughout the interior of furnace 13 and contacts wafers 35 supported on a quartz boat 37. Quartz boat 37 is a ladder arrangement of horizontal quartz wafer support structures cut in or held in place by three or more vertical rails. In a typical design, quartz boat 37 may hold between 100 and 200 wafers. The water reacts with exposed silicon on wafers 35 to produce silicon oxide layers. An exhaust port 39 draws excess water together with any gaseous carriers and reaction products out of furnace 13.
This design has certain shortcomings. Most of these shortcomings derive from the fact that a hydrogen-oxygen flame is produced. It is critically important that the position of this flame be carefully controlled so that it does not contact the quartz torch vessel wall 15 or inlet pipes 17 or 19. Thus, the torch reactor must generally assume the jug-shape or some other shape which is unlikely to contact flame 25. In addition, the size of orifice 21 must be carefully controlled so that the hydrogen flow rate is sufficiently high to prevent the flame from contacting quartz inlet pipes 17 or 19. Should the flame contact any of the quartz components described herein, the quartz my devitrify and vaporize. This introduces particulates and other contaminants to the ultra pure water source, thereby precluding generation of a high quality oxide.
Still further, many precautions must be taken to ensure that hydrogen explosions do not occur. Typical safety mechanisms include interlocks to ensure that hydrogen does not flow without oxygen also flowing into torch 11. Further, additional interlocks are provided to ensure that the ratio of hydrogen to oxygen always remains below 2:1 by volume. Typically, the hydrogen to oxygen ratio is about 1.9 to 1 by volume. Still further, the torch often includes a flame detector such as a flame detector 41 shown in FIG. 1. Such a flame detector ensures that the hydrogen and oxygen are actually reacting. If there is no flame, an explosive mixture may be forming within the torch, within the furnace, or elsewhere.
In view of the above shortcomings, an improved wet oxidation system is necessary.
The present invention provides methods and apparatus for performing wet oxidation. Wet oxidation performed in accordance with this invention does not employ a flame. Therefore, contamination due to the flame impinging on quartz components of a torch is not a problem. Flame-free generation of water is accomplished by reacting hydrogen and oxygen under conditions that do not result in ignition. In a preferred embodiment, this is accomplished by providing a diluted hydrogen stream in which molecular hydrogen is mixed with a diluent such as a noble gas or nitrogen. This use of diluted hydrogen also reduces or eliminates the danger of explosion. This can greatly simplify the apparatus design by eliminating the need for complicated interlocks, flame detectors, etc.
One aspect of the invention provides a method of forming an oxygen containing layer on a semiconductor surface. The method may be characterized as including the following sequence: (a) forming water vapor by reacting gaseous hydrogen and gaseous oxygen without generating a flame; and (b) contacting the water vapor with the semiconductor surface under conditions which form the oxygen containing layer on the semiconductor surface. The oxide may be a silicon oxide layer or a nitrogen containing silicon oxide layer, for example. The water vapor should be sufficiently pure to meet the requirements of the semiconductor device fabrication industry. Generally, this means that the gaseous hydrogen and gaseous oxygen each will have a purity of at least about 99.999% by volume.
One technique for ensuring that the water vapor is formed without generating a flame involves providing the gaseous hydrogen as a mixture of molecular hydrogen and an inert diluent. Preferably, the molecular hydrogen is present in the mixture at a concentration of at most about 10% by volume. Preferably, the diluent is molecular nitrogen or a noble gas such as argon.
Another technique for ensuring that the water vapor is formed without generating a flame involves using a hydrogen-oxygen mixture having hydrogen far in excess of the stoichiometric 2:1 molar ratio. This technique involves forming a reaction mixture of gaseous hydrogen and gaseous oxygen in a ratio of molecular hydrogen to molecular oxygen that is at least about 4:6 by volume. Any excess hydrogen present after the reaction to form water vapor is removed by flashing it off or by other suitable technique.
If the water vapor is formed in a reactor separate from the furnace, it must be transported from the reactor where it is formed to a second reactor (e.g., the furnace) where it contacts the semiconductor substrate. Alternatively, the water vapor may be formed and reacted with the semiconductor surface in a single reactor. In some embodiments, the reactor includes an injection column packed with a heated packing material over which the reactant hydrogen and oxygen gases pass. The heated packing material provides the activation energy for the flameless reaction of hydrogen and oxygen.
Another aspect of the invention provides apparatus for forming the oxygen containing layer on a semiconductor surface. The apparatus may be characterized as including at least the following features: (a) a source of flowing gaseous oxygen; (b) a source of flowing diluted gaseous hydrogen comprising molecular hydrogen at a concentration of at most about 10% by volume in an inert diluent; (c) a reactor having one or more inlets allowing the flowing gaseous oxygen and the flowing gaseous hydrogen to enter the reactor; (d) a heat source which provides heat to the reactor, thereby activating a reaction between hydrogen and oxygen to produce water vapor; and (e) a wafer support which holds a semiconductor wafer in contact with the water vapor to allow formation of the oxygen containing layer.
If the wafer support resides in a separate system, the apparatus usually includes a conduit for transporting the water vapor formed in the reactor to the furnace. Alternatively, the wafer support resides in the reactor where the water vapor is formed. The apparatus may also include an injection column disposed within the reactor and packed with a packing material, as mentioned. In this design, the one more inlets are arranged with respect to the injection column such that the flowing gaseous hydrogen and the flowing gaseous oxygen are passed to the injection column where they react to form the water vapor.
These and other features and advantages of the present invention will be described in further detail below in conjunction with the appended drawings.